Conventionally, in many cases, high strength and high toughness have been given to a machine structure part for an automobile, an industrial machine, and the like in a manner that a steel product such as a bar steel is hot forged into a part shape and then is reheated to be subjected to thermal refining of quenching and tempering. In recent years, in terms of a reduction in manufacturing cost, an omission of a thermal refining process of quenching and tempering has been promoted, and as shown in Patent Document 1 and the like, for example, there has been proposed a non-heat-treated steel to which high strength and high toughness can be given even though it remains being hot-forged. However, that both high fatigue strength and excellent machinability are accomplished is actually to be an obstacle to the application of the high strength and high toughness non-heat-treated steel to a steel part for machine structural use.
Generally, the fatigue strength relies on tensile strength, and as the tensile strength is increased, the fatigue strength is increased. On the other hand, the increase in tensile strength deteriorates the machinability. Many of the steel parts for machine structural use need to be cut after being hot forged, and the cutting cost accounts for most of the manufacturing cost of the part. The deterioration of machinability caused by the increase in tensile strength causes the significant increase in manufacturing cost of the part. Generally, when the tensile strength exceeds 1200 MPa, the machinability deteriorates significantly and the manufacturing cost is increased drastically, and thus it is practically difficult to achieve the high strength in excess of the above strength. Thus, in the parts for machine structural use, the increase in cutting cost caused by the deterioration of machinability is a bottleneck in achieving the high fatigue strength, and a technique of accomplishing both the high fatigue strength and the excellent machinability is required.
As conventional knowledge of securing machinability even though the steel part is high in strength, in Patent Document 2, for example, it has been proposed that a large amount of V is added to a steel, V carbonitride that has precipitated by an aging treatment is attached to a tool surface at the time of machining to protect the tool, which is effective for preventing tool abrasion. However, a large amount of V is needed in order to secure the machinability, and due to the steel being a high alloy, hot ductility is significantly poor. In the case when such a steel is used, there is caused a problem of occurrence of cracking and flaws to occur at the time of casting and flaws at the time of subsequent hot working, namely at the time of hot rolling of a bar steel and hot forging of a part.
As a means of accomplishing both the high fatigue strength and the excellent machinability, it is effective to improve the ratio of the fatigue strength to the tensile strength, namely an endurance ratio (the fatigue strength/the tensile strength). In Patent Document 3, for example, it has been proposed that it is effective to turn a structure mainly composed of bainite to decrease high-carbon martensite island and retained austenite in the structure. However, the endurance ratio is 0.56 or less at the most, there is a limit to increase the strength without deteriorating the machinability, and the fatigue strength and the tensile strength are both low.
Further, in Patent Document 4, for example, it has been proposed that it is effective to turn a structure into a fine ferrite-bainite structure after molding by warm forging in a temperature zone of 800 to 1050° C. and to cause V carbonitride to precipitate by a subsequent aging treatment. Generally, there is shown a tendency for the toughness to decrease when the achievement of high endurance ratio is accomplished, but by the warm forging, the ferrite-bainite structure is made fine, and thereby the toughness is improved. However, in the steel part for machine structural use requiring toughness, the improvement of toughness is small. Further, in the warm forging in the temperature zone of 800 to 1050° C., a forging load is large to thereby decrease the life of a mold significantly, and thus the production is difficult to be performed industrially.
Further, in Patent Documents 5 and 6, for example, there has been proposed a method of increasing strength by causing Ti carbide and V carbide to precipitate in a steel. However, when Ti is contained, Ti turns into nitride at high temperature preferentially to carbide, and thereby coarse Ti nitride is formed, and Ti nitride does not contribute to precipitation strengthening and further significantly decreases an impact value.